ABSTRACT
This chapter reviews recent developments in engineered optical materials, and in particular nonlinear crystals. This definition can be taken to mean conventional materials that are structurally altered to give new and enhanced optical properties. The original concept of quasi-phase-matching (QPM) dates back to the paper by Armstrong, after which various techniques such as periodic poling during crystal growth and ion diffusion to induce domain inversion were used. Periodically poled lithium niobate and its close relatives do not hold a monopoly on QPM, and indeed QPM can be carried out in materials by periodically modulating the strength of the nonlinearity rather than by inverting the sign of the nonlinearity. Having introduced the most common QPM materials, it is now appropriate to discuss the applications of these materials in optics and elsewhere. Nonlinear optics is concerned with materials in which the response of the material is nonlinear function of the incident electric fields.
